In this instructable I will be showing you how to adapt the chronographing method to give approximate rate of fire of an automatic airsoft gun.
You will need:
1. An airsoft gun
2. A computer with audio capture card ( one with a microphone input to be precise)*
3. A microphone
4. An audio capture program ( Audacity is open source, free and worked well for this application. It can be downloaded here http://audacity.sourceforge.net/)
5. A target that will cause a sound when struck and an appropriate backstop
6. As always with projectiles, A SAFE AREA TO TEST THE DEVICE AND AN APPROPRIATE BACKSTOP. ACCIDENTS HAPPEN AND SO PRECAUTIONS SHOULD ALWAYS BE TAKEN TO PREVENT INJURY.
- A device that captures audio with a high enough bit rate, preferably in .wav format could be substituted for the computer, if you can't put the computer in place, or don't have a capture card. I haven't tried this but I don't see why it couldn't work. I know there are .mp3 players out there with microphones and recording software, and usb ports are on all, if not most computers these days.
Ready to go? On to the set up
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Step 1: Setting Up
1.The target should be placed downrange a short distance (5 metres would be acceptable, depending on how loud your gun is. With a louder gun your results may be hard to decipher and so a longer distance should be used).
2.With the target in place and secured with an appropriate backstop the recording source is placed next to the target. It could be placed beside or behind the target, as long as it is close enough to catch the sound of the target being struck and will not be damaged by the incoming fire.
3.Load up your gun with enough BB's to last for a couple of seconds of full automatic fire. With some guns a full magazine is just about right (low cap. mag.).
Step 2: Firing and Principles of Operation (I'm Kinda Boring Amn't I?)
1. Begin recording with your recording equipment and fire a single test round. This will be useful later. Now fire a couple of seconds long burst of fire into your target from your firing position a few feet/ metres away (gun dependant).
2. When finished firing stop recording and save the recording to prevent loss of your data.
3. Sharing oh so many similarities with my previous Instructable, now is when we view the waveform on Audacity. In this case, we may not be able to see the waveform of the gun firing, but we don't need it and frankly it'd be better if it wasn't there cluttering the data that we want. This is why we don't shoot at a very short distance. The motor whirring and muzzle report would make life difficult for us.
So, using Audacity find the waveform of your first test shot and note it's appearance. The spike of sound is what we will need to distinguish the multiple hits from one another. We will only be using the spike at the very start of this shape as this is when the target was struck.
4.Now, move on to your long burst of automatic fire. Using your test shot as a shape guide for the shot striking the target on the waveform, find your first and last hits and take note of the times in the recording of these hits.
We now have a known length of time and by counting the number of hits we have a number of shots fired in that space of time.
Step 3: Maths!
Rate of fire = shots fired divided by time
As I have encountered it there are two general measurement of ROF and that is Rounds Per Second and Rounds Per Minute. I will show both here.
I don't have an AEG or gas auto at the moment so this will be theory for the moment.
Lets say that the gun was fired for 2.5 seconds and in that space of time 30 shots were fired.
To get the rate of fire in RPS simply divide 30 by 2.5
30/2.5 = 12 rounds per second
To get the ROF in RPM multiply the firgure for RPS by 60.
12 x 60 = 720 rounds per minute
By substituting the values you obtain these simple formulae are all you will need to figure out your rate of fire.
As with my chrono'ing method this is not perfectly accurate, particularly in the rounds per second format. By using the rounds per minute value your results will probably be more accurate. Not only that, but if you get a number like 15.762 RPS multiplying by 60 and rounding off will give a more sensible number to work with.
Step 4: Simpler Method, But Much Less Accurate.
If you don't have the recording equipment needed for this or any of the other equipment (apart from the gun of course :P) you can still get a rough idea of your ROF with a stopwatch and something to catch the BBs fired.
1. Load up your gun for a few seconds of firing. You may need mid cap. mags. for this method as low caps will run out quickly.
2. Set up the thing you are using to catch the BBs. A towel, sheet or target with net backstop could be used, as long as the BBs are caught and are not damaged by impact. You can be as close as is safe for this method so distance is not an issue.
3. Start the stopwatch and begin firing at the same time into this backstop. When the stopwatch gets to a predetermined number (2 or 3 seconds) stop firing.
4. Count the BBs caught in the towel or net and divide this number by the number of seconds fired for.
This is the ROF in rounds per second.
Again for rounds per minute multiply this number by 60.
This method is less accurate for a couple of reasons. Starting the stopwatch and firing in the same instant is difficult and even with the fastest reflexes you are likely to miss by a half a second either before or after you stop firing.
The same can be said for stopping firing when the time is over.